Considerable effort has recently been directed to the development of surface-normal semiconductor optical cavity devices such as surface-emitting lasers and asymmetric Fabry-Perot modulators. For a description of a typical such device, see, for example, "Electroabsorptive Fabry-Perot Reflection Modulators with Asymmetric Mirrors", by R. H. Yan et al, IEEE Photonics Technology Letters, Vol. 1, No. 9, pages 273-275, September 1989.
In contrast with edge-emitting or waveguide-type optical devices, surface-normal optical devices of the type described in the afore-cited article lend themselves relatively easily to the formation of two-dimensional arrays. Such arrays are useful in a variety of practical applications such as optical interconnects, laser printers and smart-pixel systems
A typical surface-normal semiconductor optical cavity device includes a multi-layer mirror structure on top of which is formed a multi-layer electroabsorptive region. Another mirror is then formed on top of the electroabsorptive region. An optical cavity, exhibiting resonance at a prospecified wavelength, is thereby formed in the device, as is well known in the art.
To achieve a good-quality surface-normal semiconductor optical device, it is essential that the thicknesses of the layers constituting the optical cavity be fabricated with good precision and accuracy. In actual devices, the thicknesses of the layers of the cavity must, for example, often be controlled over their entire extents to vary less than one percent from prescribed thickness values.
In practice, a shift in the overall thickness of the cavity layers of such a device by one percent causes a corresponding shift of one percent in the cavity resonance. For a device designed, for example, to operate at a wavelength of 850 nanometers (nm), such a thickness variation would thus cause a shift of 8.5 nm in the resonance wavelength. In many cases, such a shift is comparable to or larger than the width of the resonance of the cavity. Hence, a device fabricated with such a variation in its cavity thickness would not be acceptable for use at the designated wavelength. Accordingly, methods such as those disclosed in U.S. patent application Ser. No. 08/236,618, for example, have been developed to provide a more accurate and precise method of fabricating the multiple cavity layers of a surface-normal semiconductor optical device with minimal variation in thickness.